Ink jet recording apparatuses are widely used, being mounted on a printer, facsimile machine, or copying apparatus, as a means of recording images (including characters and symbols) on a recording medium such as paper or plastic sheets (OHP or the like) based on image information. The ink jet recording apparatuses perform recording by discharging ink droplets onto a recording medium from a recording head. They have the advantage that they can downsize a mechanism for performing recording processes and can record accurate images at high speed. Moreover, they feature low running costs and have a low noise level because of their non-impact design. In addition, they can easily record color images using inks other than black: cyan (C), magenta (M), yellow (Y), etc.
Driving sources for ink jet recording apparatuses include a carriage motor which drives a carriage carrying the recording head in the scanning direction in a reciprocating manner, a transport motor (ASF motor) which feeds the recording medium to the ink jet recording apparatus, a recovery system motor for doing head cleaning, a paper feed motor which feeds the recording medium for each print scan, etc. Conventionally, stepping motors are often used as driving sources because of low cost and the ease with which they can be controlled.
Although ink jet recording apparatuses do not produce much noise during recording because of their non-impact design as described above, DC motors are increasingly used as driving sources in order to further reduce noise. An encoder is generally used as a detector to obtain control information about DC motors (such as positional information and speed information).
FIG. 1 is a diagram modeling a principle of signal detection in an encoder. In the encoder, light emitted from an LED 101 is detected through a code wheel 102 by a detector 103, which consequently generates a signal. The code wheel 102 is patterned with slit segments 104 that transmit light from an LED 101 and segments 105 that do not transmit light alternating at predetermined intervals. The detector 103 contains photodiodes 106, 107, 108, and 109 placed at predetermined intervals, converts the light detected by the photodiodes 106, 107, 108, and 109 into respective electrical signals A (110), *A (111), B (112), and *B (113), and outputs them. Then, the electrical signals 110, 111, 112, and 113 are output as differential outputs Channel A (116) and Channel B (117) by comparators 114 and 115.
FIG. 2 shows a waveform of a differential output signal. At intersections of electrical signal A (201) and electrical signal *A (202), rectangular pulse waveform Channel A (203) is switched between a rise (High) and fall (Low). If speed is constant, intersections of electrical signal A and electrical signal *A occur at regular intervals. Thus, ideally the duty cycle (ratio between High state and Low state) of Channel A (203) is 50%. However, the duty cycle can vary due to various factors, the main one of which is sensitivity difference between photodiodes.
FIG. 3 shows a waveform of a differential output signal obtained when there is a sensitivity difference between photodiodes. The sensitivity difference between photodiodes manifests itself as a difference in electrical signal amplitude. In FIG. 3, when the amplitude of electrical signal A (301) becomes smaller than that of electrical signal *A (302), the duty ratio of Channel A (303) exceeds 50% (HD>50%) in High state and falls below 50% (HD<50%) in Low state. As can be seen from FIG. 3, the sensitivity difference between photodiodes affects the duty ratio of the output signal, but it does not affect the period of Channel A (303). Thus, the period determined from phase A and phase *A (phase B and phase *B as well) of the output signal from an encoder provides accurate information regardless of the sensitivity of photodiodes.
When detecting positional information or speed information as control information about DC motors from an encoder signal, a single-edge sampling method is used to obtain more accurate information, where the single-edge sampling method consists of counting the period from a rise to the next rise of the encoder output signal using cycle information for which high precision is ensured.
However, speed information obtained by the single-edge sampling method is updated only after the encoder output signal goes through one cycle. That is, speed information is updated at ½ the frequency of a double-edge sampling method (which detects both rises and falls of the pulses, for example, in the pulse waveform shown in FIG. 3) and only ¼ as much speed information can be obtained as when both edges of two phases Channel A and Channel *A are sampled.
Now consider, for example, carriage control for ink jet recording apparatus. First the paper is fed at high speed and then low-speed servo control is started a little before a stop position. Then, just before the target stop position, stop mode is entered and the paper is stopped at the target position. In this case, the stopping accuracy of the paper depends heavily on how the low-speed servo control is stabilized a little before the stop position. During such low-speed driving, naturally the encoder signal changes slowly and speed information is updated at long intervals in the single-edge sampling method. Thus, in servo control of a motor, any time lag between a current feature value of the controlled object and speed information fed back can make the servo operation unstable.
If the double-edge sampling method is used to solve the above problem, although speed information is updated at shorter intervals, the accuracy of detecting speed information decreases due to variations in the duty cycle for the reasons described above, making the servo operation unstable.